What a U value of 0.89 is actually worth to you
Every door company quotes a U value and almost none of them tell you what it means in euro. We ran the sum on our own product, and the honest answer is that the U value is not where the money is.
A U value is one number: how much heat passes through one square metre of something for every degree of difference between inside and outside, measured in watts. Lower is better. A solid insulated wall is around 0.2. A single glazed window is around 5.
Doors sit in between, and the number gets quoted constantly without anyone explaining what it buys you. So here is the sum, on our own product, including the part that does not flatter us.
The three numbers, and why there are three
| Variant | Declared U value | Declaration |
|---|---|---|
| Solid leaf, no glazing | ≤ 0.89 W/m²K | 72-1/03/2022/PPA |
| Glazed leaf | ≤ 1.1 W/m²K | 72-2/03/2022/PPA |
| Glazed, with sidelights | ≤ 1.3 W/m²K | 72-2/D/02/2022/PPA |
Glass is always the weak point. A four pane insulated glazing unit is excellent glass, and it is still a worse insulator than 72 mm of polyurethane. Every window you add to a door costs you thermally. That is not a defect, it is a trade, and it is your trade to make.
There is a second thing worth knowing. Our catalogue quotes 0.83 for the solid door and 0.97 for the glazed one, and the declarations say 0.89 and 1.1. Both are true. The catalogue figure is for a reference size in ideal conditions. The declared figure is the worst case across the whole size range, which is the number that has to hold up legally. When you compare quotes, make sure you are comparing declared figures against declared figures. A supplier quoting their best case against someone else's worst case is not lying, exactly, but you are not learning anything either.
All three of ours clear the SEAI threshold of 1.4 W/m²K, including the version with sidelights, which is the one people assume will fail.
Now the money
Take a standard door, 1,049 × 2,115 mm — about 2.2 m² including the frame.
Say the old door is a 1980s timber or early PVC unit at somewhere around 3.0 W/m²K. The new one is glazed, so 1.1. The difference is 1.9 W/m²K.
An Irish heating season runs roughly October to April. Call it 5,000 hours of heating at an average indoor-outdoor difference of 8 degrees. That gives 40,000 degree-hours.
1.9 W/m²K × 2.2 m² × 40,000 K·h ≈ 167 kWh a year
At gas prices that is around €18 a year. On direct electric heating, around €55. With a heat pump, back down to about €18 because the pump does three units of heat for one of electricity.
Over twenty years, in the best case for us, that is a bit over €1,000. The door costs €2,500 fitted. You do not replace a front door to save money on heating. Anyone who tells you otherwise is doing arithmetic they hope you will not check.
So what is worth paying for
Three things, in order.
The grant. €800 per door, up to two doors per property, is worth more than fifteen years of U value savings and it arrives at once. If you are replacing a door anyway, the grant is the entire financial argument. We have written it up in detail, including the condition that voids the claim.
Air tightness, which nobody quotes. The U value measures heat conducted through the door. It says nothing about air blowing around it. Our doors are air permeability class 4, which is the top of the scale under EN 12207 — and that is the number you will actually feel in the hall in January. An old door with a decent U value and a 4 mm gap along the bottom loses far more heat than the sum above, and it loses it in the form of a draught across your feet. Almost no supplier quotes this class. Ask anyway.
The things that are not heat at all. A lock you trust. A leaf that still closes with one push in ten years. A door that looks like it belongs on the house. These are the real reasons people replace front doors, and it is more honest to say so than to build a business case out of 167 kWh.
The short version
The U value matters as a threshold, not as a saving. Get under 1.4 so the grant is safe, compare declared figures rather than brochure figures, and then spend your attention on the air class, the lock and the look — because that is where the difference between a good door and a bad one actually shows up.